The quantities and types of damage to DNA purine and pyrimidine bases produced in vivo by ionizing radiation have not been completely characterized directly and related to lethal and mutagenic effects on living organisms. It is proposed to continue a research program designed to yield information about DNA base damage and its effects. The quantities and types of damage to bases in vivo will be determined in bacterial strains selected for their known sensitivities to ionizing radiation and for their genetically characterized ability to repair radiation damage. Specific products of base damage will be measured as a function of radiation dose, level of survival, and time after irradiation during the period in which radiation damage to DNA is thought to be repaired. The effects of the hydroxyl radical, oxygen, sulfhydryl, and radioprotective compounds on the types and quantities of base damage will be determined. From studies involving viral and bacterial systems which have received the most extensive genetic characterization to date, the relationships between damage to DNA bases and mutagenesis will be delineated. The studies will eventually be extended to mammalian cells. Certain in vitro studies on the effects of radiation on DNA base and sugar constituents will be carried out when the data from such studies are needed to interpret in vivo results and cannot be found in available literature. The information obtained from this project will make available for the first time, substantive data on the destruction of DNA bases as it occurs in vivo by ionizing radiation. Such data may be valuable in helping to clarify some of the results which have been obtained from research on production of DNA strand breaks by ionizing radiation. These data also will allow, for the first time, a direct assessment of the applicability of much of the previous research on the radiation chemistry of DNA and its constituents in vitro to actual in vivo damage. New insights may be provided into the chemical basis for the oxygen effect and for the modification of radiation response by various compounds with potential application in cancer radiotherapy.